Dive summary:

• It's not impossible for concrete to fail strength tests and weaken structures, but that starts an expensive and time-consuming process, so it it better for everyone if the concrete contractors makes sure the testing process did not result in a false positive.
• Among the factors to check are sampling procedures, how sample cores were made and how the concrete was cured.
• If it looks like the test results were correct, perform a low-strength investigation before starting to drill cores or tear out and replace concrete. It is possible that less drastic actions can bring up subsequent test results.

From the article:

Unnecessary low-strength concerns, rejected concrete, construction delays and expensive investigations can be avoided by properly evaluating concrete cylinder strengths. ...